Organic materials including small molecules and polymers used for electronic devices have attracted much interest due to facile preparation of the materials, low cost of substrates such as plastic films, glass and metal foils, and cost-effective processing for device fabrication. In addition, organic materials have a wide variety of properties, which can be easily adjusted by molecular structure design (Forrest 2004). Since the pioneering work of the double layer organic solar cell and the concept of a bulk heterojunction solar cell (Tang 1986; Yu 1995; Halls 1995), significant progress has been made in organic photovoltaic solar cell technology. An average increase of 1% per-year in power conversion efficiency (PCE) of organic solar cells has been achieved in the last three years.
PCE represents the efficiency of a solar cell to convert incident solar power to electric power. It can be calculated based on Eq. 1 from a current-voltage (J-V) curve:PCE=ISC×VOC×FF/Pi  (1)where VCO is the open circuit voltage which is the maximum voltage a device can produce under irradiation without any electric load in the external circuit, ISC is the short circuit current which is the maximum current a device can reach under irradiation with the electric contact of the device shorted, FF is the fill factor which is a measurement of the maximum power extraction of the device with an optimized load in the external circuit, and Pi is the incident solar power. The PCE value is directly related to the shape of the J-V curve.
In recent years, the fastest developing area in this field has been bulk heterojunction polymer solar cells (PSC), in which the heterojunction active layer comprises a semiconducting polymer as the electron donor (ED) domain and a fullerene derivative as the electron acceptor (EA) domain (Cheng 2009; Dennler 2009; Chen 2009a; Thompson 2008; Günes 2007; Mayer 2007). The high PCE of this type of device is attributed to a very large heterojunction area between the donor and acceptor domains. In this device, a photon is absorbed in the active layer and converted to an exciton, or an electron-hole pair. It is separated at the donor/acceptor interface to create an electron and a hole, which move along within the donor and acceptor domains to reach the relevant electrodes, respectively, to generate electricity. Therefore, the PCE of such a device is first dependent on the sunlight absorption efficiency of the polymer in the active layer. However, most semiconducting polymers absorb at a short wavelength. Up to now, the benchmark of polymer solar cell has been based on poly(3-hexylthiophene) (P3HT) as the donor and fullerene derivatives, such as PCBM, as the acceptor (see Scheme 1). Power conversion efficiencies (PCE) up to 4-5% have been reported (Ma 2005; Li 2007). However, this value already seems to be an upper limit since P3HT films only absorb light in a relative short wavelength region, having maximum absorption at about 510 nm with onset of absorption at about 630 nm, while the maximum photon flux region of the solar spectrum is about 700 nm. Thus, the majority solar energy cannot be used in these devices. Another drawback of P3HT is its high lying highest occupied molecular orbital (HOMO) energy levels at −4.9 eV. This limits the open-circuit voltage (VOC) to around 0.6 eV because VOC is closely related to the difference between the HOMO energy level of the donor and the lowest unoccupied molecular orbital (LUMO) energy level of the acceptor (Scharber 2006).

To overcome these problems, structure design of the polymer appears the most promising approach. Diverse organic chemistry benefits polymer design and synthesis, thus both the polymer structure and resulting properties can be effectively tuned. Introduction of alternating electron rich and electron deficient units into a conjugated polymer chain has proved to be an efficient way of reducing the band gap of the polymer due to electron delocalization, which results in a shift of light absorption of the polymer to longer wavelengths. By combining this with shorter wavelength absorption of the electron acceptor (e.g. fullerene derivatives), the bulk heterojunction active layer formed from the donor and acceptor can better cover the solar spectrum. In addition, the introduction of electron deficient units into the polymer also lowers LUMO and HOMO energy levels resulting in an increase in VOC of the device. Devices with PCE of over 6% have been reported recently by using this structure design strategy (Park 2009; Liang 2010; Liang 2009a; Chen 2009b; Hou 2009; Zhu 2007; Coffin 2009; Hoven 2010; Mühlbacher 2006; Soci 2007; Lee 2008; Peet 2007).
The cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) unit has shown strong electron donating properties in conjugated polymers, and the synthesis of solution processable polycyclopentadithiophene has been reported (Coppo 2003; Asawapirom 2001). An electron rich CPDT unit alternating with an electron deficient unit in a polymer effectively narrows the band gap of the polymer, which results in very promising properties in organic electronic devices fabricated from the polymer, especially in polymer solar cells (Zhu 2007; Coffin 2009; Hoven 2010). Recently, CPDT was copolymerized with electron deficient benzothiadiazole and the resulting poly[2,6-(4,4)-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) shows a promising PCE of 3.5% (Mühlbacher 2006; Soci 2007). This PCE was further improved to 5% by the use of processing additives (Lee 2008; Peet 2007). However, the VOC of these devices was only about 0.6 V, which limits the PCEs of the devices.
Tetrazine has a very high electron affinity and it should behave as a strong electron acceptor reducing the energy level of the HOMO of polymers containing a tetrazine unit (Clavier 2010; Saracoglu 2007; Kaim 2002). Several new heterocyclic substituted tetrazines have been reported recently (Soloducho 2003; Audebert 2004a; Audebert 2004b; Audebert 2006a; Audebert 2006b; Audebert 2009a; Audebert 2009b; Dumas-Verdes 2010), and one of them (bis[5-(2,2′-bithienyl)]-s-tetrazine (see Scheme 2) was electrochemically polymerized (Audebert 2004a). The obtained copolymer showed a significantly reduced band gap and a lower LUMO level (about 0.9 eV lower than thiophene homopolymer), indicating that the tetrazine unit has significant electron accepting ability. Various other tetrazine-containing copolymers are known in the art (Abdelwahed 2008; Sagot 2007; Topp 1996), but no solution processable tetrazine-based copolymer has ever been reported.

There still remains a need for tetrazine-containing copolymers that possess suitable properties for use in organic electronic devices, and for monomers and processes useful in the production of such copolymers.